This invention relates to Brewster angle refractometers intended for measuring the refractive indices of solids, particularly gemstones.
Refraction is the changing of the direction of an optical field passing obliquely from one material to another in which the speed of light is different. The refractive index ("n") of a material is a number indicating the ratio of the speed of light in a vacuum ("c") to the speed of light ("v") propagating through the particular material: EQU n=c/v (1)
Using optical methods, such as absorption or reflection spectroscopy, the refractive index of a material can be measured For instance, the reflectivity ("r", defined as the ratio of incident to reflected light) at the air/solid interface of a material is related to the refractive index using Fresnel's formula: EQU r=(n-1).sup.2 /(n+1).sup.2 ( 2)
Thus, measurement of the intensity of incident and reflected light allows the refractive index to be determined. The use of this particular technique to identify gemstones is described in Long, U.S. Pat. No. 3,751,162, among others. Unfortunately, this method is complicated by the fact that the intensities of two optical fields, that is, the incident and reflected fields, must be determined in order to calculate n.
Alternatively, the manner in which a single beam of light at a specific polarization is reflected (and refracted) at an air/solid interface can be used to determine the refractive index of the solid. Specifically, for an incident optical field, there is a particular angle of incidence (relative to the normal vector of the surface), called the Brewster angle, which is related to the refractive index of a material. At this angle, the reflection coefficient of light polarized parallel to the plane of incidence (that is, the plane containing the incident and reflected fields) is zero. Thus, if the incident light is non-polarized and impinges on the material at the Brewster angle, the light reflected from the solid will be polarized in the plane perpendicular to the plane of incidence. If the incident light is polarized parallel to the plane of incidence, the intensity of the reflected light will be theoretically zero at the Brewster angle.
Determination of the refractive index can be used to identify a particular material. For example, at 589 nm, the refractive index of diamond is 2.42, whereas cubic zirconia, a manufactured diamond simulant, has a refractive index of about 2.18. Accurate determination of the refractive index can be used to differentiate one gem from another, and allows imitation materials to be distinguished from gemstones.